The present invention relates to a color picture tube having an improved inline electron gun, and particularly to an improvement in the gun for obtaining equal raster sizes (also called coma correction) within the tube, without severely distorting the electron beams.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths to a point or small area of convergence near the tube screen.
A problem that exists in a color picture tube having an inline gun is coma distortion, wherein the sizes of the electron beam rasters scanned on the screen by an external magnetic deflection yoke are different because of the eccentricity of the positions of the two outer beams with respect to the center of the yoke. This coma problem has been solved in the prior art by including various shaped magnetically permeable members adjacent to or around the electron beam paths in a fringe portion of the yoke deflection field. For example, Hughes, U.S. Pat. No. 3,873,879, issued Mar. 25, 1975, teaches the use of small disc-shaped enhancement elements above and below the center beam and ring or washer-shaped shunts around the two outer beams. The enhancement elements concentrate the vertically extending horizontal deflection field lines at the center beam path. The shunts completely surround the outer beams and bypass fringe portions of both vertical and horizontal deflection fields around the outer beams. The shunts also concentrate the horizontally extending vertical deflection field at the center beam path, thereby enhancing the vertical deflection of the center beam. If further enhancement of the vertical deflection of the center beam is required, the outer diameter of the washer-shaped shunts can be enlarged to collect more of the vertical deflection field. However, there is a limit to the maximum size shunt diameter. If the shunts are made too large, they will begin to extend into the area of the center beam. Recently, a yoke has been developed that requires a very large vertical coma correction. It is not possible to use washer-shaped shunts to provide the required coma correction, because the shunts would overlap the center beam. Although coma correction could be provided by the use of other types of shunts, such as C-shaped shunts or D-shaped shunts, the lack of symmetry of such shunts can severely distort the electron beams. Therefore, there is a need for a shunt design that will provide the large coma correction required by the new yoke and will not severely distort the electron beams.